Pile tester

ABSTRACT

A self-contained unit has a probe (N) urgeable against a structure to be tested, a shock assembly (H, I, J, F, M) for applying a shock to the structure through the probe (N), and a transducer (A, B, C) for receiving resulting vibrations from the structure through the probe (N) and providing electrical output data. The unit also includes a memory (b) for storing the data. The unit has a tubular body (E) in which the transducer (A, B, C) is receivable. The transducer (A, B, C) is linked to a weight (H) and springs (G, I) such that urging the probe (N) against a structure progressively compresses springs (G, I), drives the enclosure (A, B, C) into the body (E) and raises the weight (H) until the weight is suddenly released and hits the transducer (C) to supply the shock.

TECHNICAL FIELD

This invention relates to apparatus and methods for the evaluation ofthe integrity of building structures and the like, particularlyfoundation piles for buildings and structures.

BACKGROUND ART

It is known to apply a mechanical shock to a pile, detect the resultingvibrations of the pile, and analyse them to provide data about the pile.Such methods are known variously as `acoustic`, `sonic`, or `transientdynamic` pile survey. Essentially, the method measures the pile-head'svertical motions in response to a mechanical shock. The shock may begenerated by a manually-delivered blow downwards on the pile-head,sometimes via a `load cell` which measures the force of the initialshock, and the subsequent vertical motions of the pile-head, or it maybe that the concrete is struck directly and the resulting motions areregistered by a velocity or acceleration transducer firmly attached tothe concrete nearby the shock position.

The shock causes the concrete to be momentarily compressed, and verticalmotions of waveform approximating a decaying sinusoid follow the blow,as measured at the pile-head. In descending the column of the pile,reflection of proportions of the shock wave occur wherever the columnvaries in cross-sectional area, or in density. On returning to thepile-head such reflections `interfere` with the decaying sinusoidmotions there, and interpretation of this resultant complex waveformenables predictions as to the existence of defects in the pile column,and confirmation or otherwise that it is of the correct length. Thetechnique is now well-established in the Construction Industry.

There are three essential conditions for the acquisition of the pileshock vibration data:

1. the vibration transducer must be held in firm contact with theconcrete of the pile-head

2. a shock (or `impulse`) must occur

3. a record of the vertical vibration motions of the transducer (andthus the pile-head) must be made

In addition, a means should exist for recording the identity (its sitenumber) of the pile under test.

In many site circumstances, access to the piles may not bestraightforward. They may be relatively distant from a position to whicha vehicle can be conveniently brought, or the pile-heads may be coveredin mud, water or other site debris, and site plant and other obstaclesmay be inimical to the presence of the recording apparatus and thenormal preparations necessary for the tests to proceed.

Existing methods are slow in execution, and may be difficult orimpossible to carry out in the face of such usual construction siteobstacles as mud, flooding, deep trenching, shuttering, site traffic andso on.

DISCLOSURE OF THE INVENTION

It is an objection of this invention to provide an apparatus that canacquire the necessary information to enable an evaluation of piles'soundness and fitness for their purpose in as short a time as possibleand with the minimum possible disruption of the normal site activities,and with minimum hazard to the pile surveyor and other site personnel.

According to the present invention there is provided apparatus for usein testing structures, comprising a self-contained unit comprising shockmeans for delivering a mechanical shock of predetermined magnitude to astructure; transducer means for sensing vibrations of a structure causedby said shock and producing a corresponding electrical signal; andmemory means coupled to said transducer means for storing electricalsignals from it;

said shock means comprising potential energy storage means adapted to begradually charged and suddenly discharged to deliver said mechanicalshock; wherein the apparatus includes a probe arranged to transmit saidshock from said shock means to a structure, and to transmit saidvibrations to said transducer means; and the probe is arranged to effectsaid gradual charging by a force acting on the probe when the probe isurged against the structure to be tested.

Suitably the shock means includes a mass and means whereby it is guidedto strike, with constant force, a rigid assembly containing thetransducer, causing the transducer to produce a voltage outputrepresenting its mechanical displacements along the axis of theassembly.

In the development of preferred embodiments of the invention, accounthas been taken of all the obstacles to the testing procedures normallyencountered on construction sites, and an apparatus has been developedwhich can be deployed rapidly and reliably, and with minimal or nointerference with the usual site activities and traffic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic axial section through an apparatus embodying theinvention; and

FIG. 2 is a view on a larger scale of portions of the drop-weightmechanism of the FIG. 1 embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

The illustrated apparatus has a cylindrical outer sleeve E. Adjacent itslower end there are a plurality of longitudinally extending guide slots.A transducer carrying chamber B has the form of a cylindrical cup, whoseinterior cavity opens into the interior of the sleeve E. Within the cupcavity there is an electromagnetic velocity, acceleration ordisplacement transducer assembly A having transducer terminals R, S. Ashaft passes threadedly through an aperture in the base of the cup andbears a chuck D in which a replaceable steel probe or contact rod Nselected to be of suitable dimensions for the job in hand is gripped.The mouth of the cup is threaded to receive a lid C. Transverse guidepins T (one of which can be seen in FIG. 1) extend outwardly from thecup, through respective guide slots. The chamber B is telescopicallyreceivable within the sleeve, its maximum travel being indicated by anarrow U.

A push-rod F extends within the sleeve E. Its lower end rests on the lidC of the chamber B. Its upper end and associated parts of a drop-weightmechanism are shown in FIG. 2. A drop-weight lifting lever M ispivotally mounted inside the sleeve E adjacent the top of the pushrod Fso as to be pivotable upwardly from the configuration shown in brokenlines in FIG. 1 in which it extends downwardly at an angle to thesleeve's axis.

The lever M has a slot bounded adjacent the pivot by a stepped surfaceformed of a rod displacement profile Q, a ratchet profile P and atransverse portion W. The rod displacement profile Q extends from thelongitudinal axis of the lever M to the lower edge thereof, angledslightly towards the distal end of the lever M. The ratchet profile Pextends from the axial end of the displacement profile Q along thelongitudinal axis, away from the pivot. The transverse portion W extendsfrom the ratchet profile P transversely to the longitudinal axis.

A spring X (FIG. 2) attached to a collar (FIG. 2) on the rod F urges therod leftwards (with reference to the orientation shown in FIGS. 1 and 2)so that its top end tends to contact the stepped surface P, Q, W.

The distal end of the lever M has a line anchored to it. This carries adrop-weight H which is displaceable within a drop-weight guide cylinderJ. At upper and intermediate regions of the sleeve E, there are internaldrop-weight guide spacing rings K, L which hold the guide cylinder Jcentrally within the sleeve E. A transducer carrying-chamber compressionspring G is engaged between the intermediate ring K and the chamber lidC. A drop-weight acceleration spring I is engaged between the top of thedrop-weight H and the upper spacing ring L.

In use, pressure is applied downwards on the enclosing sleeve E,compressing transducer chamber compression spring G, which forcestransducer chamber B, C against the concrete of the pile-head by way ofthe contact rod N held in the chuck D. The pressure might alternativelybe maintained by pneumatic, magnetic or other known means of exertingpressure, acting on the transducer chamber.

As the transducer chamber rises in the tube, push-rod F engages theratchet contour of the drop-weight lifting lever M, raising thedrop-weight lifting lever M, which raises the drop-weight H against thedrop-weight accelerating spring I. As the lever approaches thehorizontal, the push-rod is forced away from the ratchet P by theprofile Q (FIG. 2). The drop-weight is released and is accelerated byspring I and gravity towards the transducer cover C, striking it. Theweight's travel is indicated by arrow Y. The force is transmitted to theconcrete via the chamber sides and base B, chuck D and contact-rod N;the whole assembly being held during this time in firm contact with theconcrete by spring G. Alternative simple hoist and release mechanismscould feature toothed wheels and/or rods, and the acceleration springmight be replaced by pneumatic or magnetic means of exerting pressure,or it might be omitted altogether, leaving acceleration of the mass tobe caused by gravity alone.

The transducer assembly A will produce at its terminals R, S analternating voltage proportional to the vibration motions of thepile-head along the axis of the chamber and the tube in response to theshock. Thus a single downward motion of the complete assembly willresult in a voltage waveform representing the pile-head vibrationresponse to a shock at the tip of rod N.

At the top of tube E an enclosure Z may be mounted containingsolid-state memory circuitry, numerical keyboard, voltage-levelattenuator and liquid crystal display, capable of storing by knowndigital means vibration motion representations of up to 120 shocks, andtransferring these at a later time to a computer for analysis,processing and paper print-out. For example, such a print-out maycomprise an oscillation trace, a listing of data calculated by thecomputer (e.g. Toe reflection time, decay rate of oscillation, andestimated length of pile), and a listing of identification data at leastsome of which are input on site, using the keyboard of the apparatus.Such data may include some or all of the pile number, the impulsenumber, identification of the client and site, and the date and time.

The enclosure Z on the apparatus may house batteries a, a memory bemploying removable memory cards, a display screen c, a keyboard d, anda multiposition switch e. Handles f for operating the device may extendon either side.

The switch e may be a four-position key-operated switch, for selectingthe modes: "off", "review stored data", "record" and "erase memorycard". The keyboard is used to enter the pile numbers, adjust thesensitivity (rarely needed), and review the stored data and waveforms.The overall weight of the unit is desirably below 14 lbs 7(kg).

With its low weight and rapid sounding capacity the apparatus cancomplete surveys of fifty or a hundred piles in one or two hours. Evenwhere there are obstructions on the site and the piles have not beenprepared for testing, site hold-ups are negligible. Because theapparatus makes contact with the pile-head through its single steelprobe, it can easily operate through reinforcing bars and cages andoverlying water, mud and loose rubble. The lightness of the unit alsomeans that it can easily be carried hundreds of meters to piles wherevehicles cannot reach.

I claim:
 1. Apparatus for testing foundation piles comprising aself-contained unit which comprises:an elongate body; laterallyprojecting handle means at an upper region of the body; a transducermeans for sensing vibration and producing corresponding electricalsignals; memory means coupled to said transducer means for storingelectrical signals from said transducer means; a carrier for saidtransducer means, said carrier being mounted to said body so that saidbody is longitudinally displaceable relative to said carrier; anelongate probe coupled to said carrier and projecting downwardly fromthe carrier, said probe projecting downwardly from said body to anextent that is variable by displacing said body relative to saidcarrier; and shock means mounted to said body for delivering apre-determined mechanical shock to said probe; said shock meanscomprising potential energy storage means including lever means coupledto said body and said carrier so that said relative displacement of thebody causes the lever means to pivot; and a weight coupled to the levermeans so as to be lifted thereby, and the arrangement being such thatdownward displacement of the body relative to the carrier causes upwarddisplacement of the weight relative to the carrier and to the body; saidshock means further including release means for suddenly releasing theweight after the weight has been raised a predetermined distance so asto impart a mechanical shock to said probe; the arrangement being suchthat the apparatus can be held by the handle means and the probe canthen be urged downwardly against a foundation pile, with continuingdownward pressure producing downward displacement of the body relativeto the carrier and a consequent raising of the weight, and a subsequentrelease of the weight by the releasing means so as to administer a shockwhich is transmitted by means of the probe to the foundation pile; theshock and subsequent vibrations being sensed by the transducer meanswhich transmits corresponding signals to the memory means for storing bysaid memory means.
 2. Apparatus according to claim 1 wherein saidpotential energy storage means further includes spring means coupled tothe weight so as to urge the weight downwardly so that the spring meansis energized as the weight is lifted.
 3. Apparatus according to claim 1wherein said lever means comprises a lever pivoted at a pivot to saidbody and coupled to said weight at a region remote from the pivot; and atransmission rod extending between said carrier and a region of saidlever close to said pivot such that upward movement of said rod tends tocause upward pivoting of said lever; and wherein said release meanscomprises a shaped portion of the lever which is arranged to push therod out of engagement with the lever which the lever has pivotedupwardly to a predetermined extent.